Revert unconditional emission of generic collision diagnostic

Emission of this diagnostic became unconditional in 8e83a57, prior to
which it was sometimes suppressed in rustdoc, in order that colliding
lifetimes would resolve to `LifetimeRes::Error` (which now requires an
`ErrorGuaranteed`).  However, that case was already caught and for
lifetime errors the code was already unreachable.

Author: eggyal

compiler/rustc_resolve/src/late.rs

@@ -3068,13 +3068,9 @@ impl<'a, 'ast, 'ra, 'tcx> LateResolutionVisitor<'a, 'ast, 'ra, 'tcx> {
                     Entry::Occupied(entry) => {
                         let span = *entry.get();
                         let err = ResolutionError::NameAlreadyUsedInParameterList(ident, span);
-                        let guar = self.r.report_error(param.ident.span, err);
+                        let guar = self.report_error(param.ident.span, err);
                         let rib = match param.kind {
-                            GenericParamKind::Lifetime => {
-                                // Record lifetime res, so lowering knows there is something fishy.
-                                self.record_lifetime_param(param.id, LifetimeRes::Error(guar));
-                                continue;
-                            }
+                            GenericParamKind::Lifetime => unreachable!(), // handled by `if let` above
                             GenericParamKind::Type { .. } => &mut function_type_rib,
                             GenericParamKind::Const { .. } => &mut function_value_rib,
                         };